New auto pilot install

[alctel]

I did a search but could only find older discussions, or about Raymarine stuff.

Boat is a 1980 Hunter 36, 13500lbs unloaded, 36 foot.

I have an NSS8 with the B&G wind sensor and simrad GS25 GPS and am looking to install an autopilot. This is what I have on my shopping list so far

- RC42 rate compass (although I hear an Airmar one is better? Is that true?)
- RF25N rudder sensor
- AC12 Autopilot CPU (I don't need the AC42 I don't think)
- some kind of drive unit - RPU80? (This is the part I'm most unsure about)

I have wheel steering but great access to the rudder shaft and plan to mount a mini-tiller attachement for the AP to connect onto.

This is going to be in addition to a windvane I am building

Any comments or additions I've forgotten?

[colemj]

The RPU80 is a hydraulic pump meant to go in-line with a hydraulic steering system. I presume yours is not one of these, so you will need to chose a linear drive instead.

The Airmar compass is a better one, but much more expensive. One advantage of staying with the RC42 is that it is setup and calibrated right from your B&G menu. Having said that, if I were to choose again, I would strongly consider the Airmar.

Mark

[Frankly]

Not sure about the room in the steering pedestal, but if you can install a drive that connects to the steering shaft vice the rudder shaft your AP system will consume a lot less energy. May not be a big issue in your case, and clearances in Edson pedestals are not much (with shift and throttle), but might be worth a look/ see before final decision.

______________________

"If at first you don't succeed, sky diving is probably not for you."

[alctel]

Originally Posted by colemj

The RPU80 is a hydraulic pump meant to go in-line with a hydraulic steering system. I presume yours is not one of these, so you will need to chose a linear drive instead.

The Airmar compass is a better one, but much more expensive. One advantage of staying with the RC42 is that it is setup and calibrated right from your B&G menu. Having said that, if I were to choose again, I would strongly consider the Airmar.

Mark

Yeah it's a few hundred more and I've heard anecdotally that it's better, just not why. I think I completely misunderstood the drive type I need, ugh

Originally Posted by Frankly

Not sure about the room in the steering pedestal, but if you can install a drive that connects to the steering shaft vice the rudder shaft your AP system will consume a lot less energy. May not be a big issue in your case, and clearances in Edson pedestals are not much (with shift and throttle), but might be worth a look/ see before final decision.

______________________

"If at first you don't succeed, sky diving is probably not for you."

I'm not quite sure what you mean with this, could you explain a bit further? I was going to attach a 'tiller' directly onto the rudder shaft

[Vipe6]

Take a look at a bcalmxp course computer with AHRS-sensor coupled to a Jefa drive, in your case a Jefa direct drive or depending on your steering system a transmission or sprocket drive unit.

[alctel]

Originally Posted by alctel

I'm not quite sure what you mean with this, could you explain a bit further? I was going to attach a 'tiller' directly onto the rudder shaft

It's ok, figured it out. You mean a rotary type system, right?

re: drive unit, it looks like I'd have to choose between a linear drive like the HLD350 Drive Unit, a direct drive like the DD15 or a rotary drive

Not quite sure what the difference between the Direct and linear systems are, they look to work the same way?

There is also the SD10 Drive Unit but am I right in thinking that's a bit crap?

[Frankly]

With the general caveat of "YMMV" the drive from the wheel shaft logic goes like this.

For most sailboats sailing in normal conditions (reasonably balanced and not sailing in gale force conditions) most of the energy consumed by the AP drive is used to overcome the polar moment of inertia of the steering/ rudder system. Small rudder movements usually corresponds to small rudder hydrodynamic forces. If the AP is doing its job rudder corrections are small.

Torques pass through a gearing system as gear ratio, polar moments of inertia pass through as the square of the gear ratio. For purposes of discussion assume the gearing ratio(wheel to rudder) is 9 to 1. When you connect directly to the rudder shaft the polar moments of the rudder assy is whatever it is, but backdriving the steering wheel you are driving the polar moment of inertia at a 1 to 81 disadvantage. Now drive from the wheel shaft, the wheel polar moment is what it is, but the rudder assy polar moment is now reduced by the 81 to 1 ratio.

This is why the wheel mounted AP guys (RM wheelpilots, CPT, IPs with thru the legs) all report low energy usage and the rudder shaft drive guys complain about the AHs their AP consumes. The steering shaft drive on my IP 32 was fused at 5 amps (and never blew), now my X5 has current limit built in so the fuse is no longer needed. When the AP's is driving all I see are little flicks of the ammeter as the course corrections happen. Most of my energy consumption is to keep the beer cold.

If you can make it work, the steering shaft is the place to apply the force.


________________________


"some days you are the dog, and some days the tree"

[aluijten]

Originally Posted by Frankly

If you can make it work, the steering shaft is the place to apply the force.

True from energy consumption point of view, but not from responsiveness point of view. Direct drive AP has great performance and will keep boat on course much longer then the pedestal add-on models.
This is also why the direct drive model can be used for boats with higher displacement.
To lower the energy consumption you can have a clutch-mechanism on the wheel itself to lower the inertia involved mass or use a fancy lightweight wheel out of carbon fiber.

Avoiding a linear drive is a good idea as well as mechanisms with only rotation are much more reliable then linear movements (at comparable cost).